1. Field of the Invention
The present invention relates to an image bearing member for bearing a static image and/or a toner image, and, more particularly, to an image bearing member exhibiting excellent durability and an apparatus which uses the same.
2. Related Art of the Invention
The image bearing members are usually classified into electrophotographic photosensitive members and other image bearing members. For example, they are exemplified by intermediate transfer members for a color copying machine in which multiple cycle of transfer operations must be performed and members such as electrostatic recording members for bearing a static image and/or a toner image.
The electrophotographic photosensitive members are structured variously in accordance with the desired characteristics or the kind of the employed electrophotographic process. As a typical electrophotographic photosensitive member used widely, there is a photosensitive member structured in such a manner that the photoconductive layer is formed on a supporting member and another photosensitive member structured in such a manner that a protection layer is formed on its surface. The photosensitive member comprising the supporting member and the photoconductive layer is used to form an image by the ordinary electrophotographic process, that is, by charging image exposure, development and, if necessary a transfer process. The protection layer of the photosensitive member having the protection layer is used to protect the photoconductive layer, improve the mechanical strength of the photosensitive member and improve the dark damping characteristics. As an alternative to this, the above-described protection layer is used so as to be subjected to a specific electrophotographic process. The above-described specific electrophotographic process is exemplified by a method which utilizes the movement of a charge, which has been injected from the supporting member at the time of the charging process, into the position between the protection layer and the photoconductive layer. The above-described method is, as disclosed in Japanese Patent Publication No. 42-23910 and Japanese Patent Publication No. 43-24748, exemplified by a method in which the static image is formed by a primary charge and a secondary charge the polarity of which opposes the polarity of the primary charge or the AC electricity removal, the simultaneous image exposure and the entire surface exposure. In the above-described method, the image exposure may be performed before or after the secondary charge or the AC electricity removal process. Furthermore, the entire surface exposure may be omitted.
Another method disclosed in U.S. Pat. No. 3,041,167 is known.
The electrophotographic photosensitive member is subjected to a predetermined electrophotographic process so that a static image is formed, the static image being then developed so that it is converted into a visible image.
Other typical image bearing members will now be described.
(1) A static image formed on an electrophotographic photosensitive member is transferred to another image bearing member to perform the development process. Then, the tone image is transferred to the recording member.
(2) As the other electrophotographic process in which a static image is formed on another image bearing member to correspond to the static image formed on the electrophotographic photosensitive member, a process is exemplified in which a static image is, by a predetermined electrophotographic process, formed on a screen-like electrophotographic photosensitive member having a multiplicity of small openings. Furthermore, the other image bearing member is subjected to a corona charging process via the above-described static image so that the corona ion flow is modulated. As a result, a static image is formed on the other image bearing member before it is toner-developed so as to be transferred to the recording member, causing a final image to be formed.
(3) Another electrophotographic process is arranged in such a manner that the toner image formed on the electrophotographic photosensitive member or the other image bearing member is not directly transferred to the recording member but the same is transferred to another image bearing member. Then, toner and the image are transferred from the above-described image bearing member to the recording member before they are fixed. The above-described process is particularly effective when a color image is formed or a high-speed copying operation is performed. Since the major portion of the recording members comprise paper or film which has flexibility, a color image, the position of which has been further accurately aligned, can be formed by method in which images of the three colors are transferred to an image bearing member which can be made of material which cannot easily be deformed before the images of the three colors are collectively transferred to a recording member. This is compared to a method in which the three color images are transferred to the recording member while accurately aligning their positions. Furthermore, the copying speed can be raised by a method in which the toner image is transferred to the recording member via the image bearing member.
(4) Another process is exemplified in which an electric signal is applied to a multi-probe electrode to form a static image which corresponds to the electric signal on the surface of the image bearing member before the static image is developed to form a desired image.
The image bearing member for use in the static-image forming processes (1) to (4) does not require a photoconductive layer.
As described above, a variety of members each having a surface layer is composed of an insulating layer such as the electrophotographic photosensitive member the surface layer of which is a protection layer or a photoconductive layer which is used as the image bearing member on which a static image or a toner image is formed.
The image bearing member must have electric characteristics which correspond to the subject recording process. Among others, durability is an important characteristic for the image bearing member, durability being a characteristic which is required in a case where the image bearing member is repeatedly used.
That is, the image bearing member must, of course, have a predetermined sensitivity level, the electric characteristics and optical characteristics of which respectively correspond to the subject electrophotographic process. In particular, since electric and mechanical external force is directly applied to the surface layer of the photosensitive member which can be used repeatedly in such processes as the corona charging, toner development, transference to paper and cleaning process, the photosensitive member of this type must have sufficient durability against the above-described external force. Specifically, the photosensitive member must have the durability against deterioration in the sensitivity, reduction in the potential and increase in the residual potential due to the deterioration caused from to the presence of ozone or NO.sub.x generated at the time of the corona charging process and surface wear and damage due to the abrasion.
In particular, the cleaning facility is an important factor which can be improved by reducing frictional resistance.
Since the surface layer of the image bearing member is mainly made of a resin and photoconductive material, the performance of the resin serves as the important role. Therefore, there is a desire for an excellent resin which is able to meet the above-described characteristics. Recently, a polycarbonate resin has been employed as the binder (a binding agent) for the surface layer because it is able to meet the above-described characteristics.
Durability of thousands to ten thousand of paper sheets realized in a case where the acrylic resin is used can be improved to a durability of fifty thousand to a hundred thousand sheets by using the polycarbonate resin. However, the durability of three hundred thousand to a million sheets realized by using Se or a-Si (amorphous Si) which is inorganic photosensitive material cannot be realized.
Therefore, a multiplicity of studies have been conducted regarding technology in which the protection layer is formed by adding a conventional resin or a resin containing fluorine. However, there arises a problem in that the residual potential (Vr) is undesirably raised or the sensitivity is deteriorated due to the repeated use of the image bearing member because of the presence of a layer of the photosensitive layer in which the charge is not moved. The above-described problem can be overcome by reducing the thickness of the protection layer to, for example, 2 to 3 .mu.m or less. However, the conventional resin encounters a problem in that the wear due to the repeated use cannot be prevented, that is, the durability cannot be improved.
In a case where a resin, to which polytetrafluoroethylene (hereinafter sometimes abbreviated to "PTFE") is added, is used to form the protection layer, a soft resin must be used in order to cause the PTFE to exhibit its excellent cleaning facility. The reason for this lies in that the fresh PTFE must be allowed to appear outside by cutting the surface by a small quantity at each use of the photosensitive material because the effect of the PTFE cannot be obtained if a hard binder is used. If the soft binder is used, the durability of the protection layer can be improved due to the effect of the PTFE. However, a flaw can easily be formed due to the friction of the cleaning blade or a crack (separation of the film) can easily be formed due to the shock because the film is soft. Furthermore, if the front or the rear end portion of the transfer paper comes in contact with the image bearing member, the contact portion of the image bearing member is damaged, causing a problem of a black line or the like to take place. Furthermore, the conventional problems occurring in the conventional protection layer such as the increase in the residual potential and the deterioration in the sensitivity due to the repeated use cannot be overcome.
It might be considered feasible to employ a hard resin in order to improve the wear resistance. However, the coefficient of skin friction is excessively enlarged in comparison to a case in which polycarbonate or the like is used to form the surface layer. In this case, there arises a problem in that excellent cleaning facility cannot easily be obtained.